<!DOCTYPE html>
<!--[if lt IE 7]>      <html class="no-js lt-ie9 lt-ie8 lt-ie7"> <![endif]-->
<!--[if IE 7]>         <html class="no-js lt-ie9 lt-ie8"> <![endif]-->
<!--[if IE 8]>         <html class="no-js lt-ie9"> <![endif]-->
<!--[if gt IE 8]>      <html class="no-js"> <!--<![endif]-->
<html>
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title></title>
    <meta name="description" content="">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="">
    <style>
      html {
        font-size: clamp(14px, 5vw, 28px);
        font-size: clamp (1rem, 1.3333333, 2rem);
      }
      .aaa {
        width: 10rem;
        height: 10rem;
        background-color: #ccc;
        font-size: 1rem;
      }

      .bbb {
        width: 2.6667vw;
        height: calc(2 * 2.6667vw);
        background-color: red;
        font-size: 1rem;
      }
    </style>
  </head>
  <body>
    <div class="aaa">AAAA</div>
    <div class="bbb"></div>
    
  </body>
</html>
<script>
  let bbb = document.querySelector('.bbb')
  console.log( bbb.getBoundingClientRect().width );
</script>